Generally, in sewing machines to be used for manufacturing sewn products and for weaving various fabrics, an article is sewn by combination of two threads classified into a bottom thread and an upper thread and by the operation of a needle.
The bottom thread is wound around a bobbin and is supplied by a bottom thread supply device including the bobbin. A representative example of such a conventional bottom thread supply device is shown in FIG. 8.
Furthermore, FIG. 9 is an assembled view showing the conventional bottom thread supply device. FIGS. 10 and 11 are rear views showing a conventional rotary hook and shuttle hook, respectively.
As shown in the drawings, the conventional bottom thread supply device includes a rotary hook 10, a shuttle hook 20, a bobbin 30, around which a bottom thread 35 is wound, and a bobbin case 40. The above-mentioned components have the following functions.
A rotating shaft 17 of the rotary hook 10 rotates along with a main body 15 of the rotary hook 10. Thereby, the bottom thread 35 is unwound from the bobbin 30, and inserted in both the shuttle hook 20 and the bobbin case 40, which are stationary.
However, the conventional sewing machine have no a remaining thread detection device. Therefore, during a sewing process, a user cannot know whether bottom thread 35 remains, thus resulting in unproductive sewing. This problem causes defective article, thus resulting in economic loss.
To solve the above-mentioned problems, a variety of techniques, which let a user know an appropriate time to replace a bottom thread 35 with another, have been proposed. Among them, a detecting device, which detects whether bottom thread 35 remains in a bobbin 30 using an optical sensor that detects light reflected by the bottom thread, was proposed in PCT Publication No. WO88/07099. However, due to mobility and variation in thickness of the bottom thread, there is a disadvantage of reduced accuracy.
Furthermore, in Korean Patent Laid-open Publication No. 96-0034515, whether remaining thread exists is determined using the number of rotations of a bobbin. However, it is difficult to apply this technique to a typical bottom thread supply device. Thus, there is a disadvantage of requirement of additional installation cost.
In an effort to overcome the problems experienced with the conventional techniques, the inventor of the present invention proposed several techniques in Korean Patent Application No. 10-2003-0020289, and Korean Utility Model Registration No. 20-0274005-0000, No. 20-0269008-0000 and No. 20-0330729-0000.
In Korean Utility Model Registration No. 20-0269008-0000, which is entitled ‘SEWING MACHINE HAVING SHUTTLE THREAD DETECTION SENSOR’, both a light receiving sensor and an elastic electrode, which couples an electrode of the light receiving sensor to a rotary hook, are provided in a shaft of a shuttle hook. A light source, which emits light, is disposed adjacent to a bottom thread supply device.
If no more bottom thread exists, the light receiving sensor detects emitted light and transmits a detection signal through the elastic electrode provided between the shaft of the shuttle hook and the rotary hook, thus letting a user know that there is no remaining thread.
However, because the elastic electrode is affected by physical force occurring during a process of unwinding the bottom thread from the bobbin, in the case of continued use, there is a disadvantage in that fine modulation is required.
To supplement the above-mentioned disadvantage, a bottom thread detection device of a sewing machine using an optical fiber was proposed in Korean Utility Model Registration No. 20-0330729-0000, which was filed by the inventor of the present invention and is characterized in that a light receiving sensor, provided in a shaft of a shuttle hook, is inserted at a central portion in a rotary hook, and an optical fiber is provided in the shaft of the shuttle hook so as to guide incident light to the light receiving sensor.
In this technique, because the light receiving sensor is disposed in the main body of the rotary hook, the elastic electrode, used in No. 20-0269008-0000, is not required. Therefore, the shaft of the shuttle hook does not physically contact the rotary hook, so that the installation is simplified. Furthermore, this technique solves the problem of fine modulation experienced with No. 20-0269008-0000.
However, when incident light that enters the shaft of the shuttle hook is transmitted to the light receiving sensor through the optical fiber in the shaft, due to increased light loss, there is a probability of a malfunction in which the incident light is not detected by the light receiving sensor. Furthermore, there is a disadvantage in which the light receiving sensor may operate erroneously due to scattered reflection of light.
As well, in order to increase detection efficiency of the light receiving sensor, superior light collection efficiency and light linearity of the light source, which emits light, have been required.